Hard heartbeat around ex

I dated my ex for a while around a year and a half ago. We used to be very close. We talked alot on the phone even though we saw eachother almost everyday. Some conversations would go on for hours. I swear we even had a 7 hour conversation once while we were both tied up with errand after a long day. Anyway, this was quite a while back. Since then we don't talk much and she's seeing someone else and they've been going out for a couple months. The thing is whenever I see her I start getting these weird heart palpitations. It's like all of a sudden my heart start beating very hard. I have no control of it, it just seems to happen. Sometimes when I just spot her in the street and she's walking towards me. The reason I'm posting on the forum is because I've never had this happen to me before and I find it odd frankly. It just doesn't seem normal. So my question is, what could be the reason or cause of this? Has this or does it happen to anyone else. Any feedback would be appreciated.

Thanks in advance